A multilayer ceramic capacitor is generally formed as described below.
First, ceramic green sheets are prepared, each having a conductive material on a surface thereof to be formed into an interior electrode which has a desired pattern, and each containing a powdered dielectric ceramic starting material. As the dielectric ceramic, for example, a ceramic primarily composed of BaTiO3 is used.
Next, ceramic green sheets including the above-described ceramic green sheets provided with the conductive material are laminated to each other and are then thermally bonded to each other, thereby forming an integrated green laminate.
Next, this green laminate is fired, thereby obtaining a sintered laminate. Inside this laminate, the interior electrodes are formed using the conductive material described above.
Subsequently, on exterior surfaces of the laminate, exterior electrodes are formed to be electrically connected to specified interior electrodes. The exterior electrodes are each formed, for example, by applying a conductive paste containing a powdered conductive metal and a glass frit onto the exterior surfaces of the laminate, followed by baking.
As described above, the multilayer capacitor is formed.
In order to reduce the cost for manufacturing a multilayer ceramic capacitor as low as possible, a relatively inexpensive base metal such as nickel or copper has been frequently used in recent years as the conductive material described above for forming the interior electrodes. When a multilayer ceramic capacitor having interior electrodes made of a base metal is manufactured, firing must be performed in a neutral or a reducing atmosphere in order to prevent the base metal from being oxidized in firing.
However, by firing in a neutral or a reducing atmosphere, in general, a ceramic composed, for example, of barium titanate is extremely reduced, and as a result, a problem may arise in that the ceramic becomes semiconductive.
For solving the problem described above, in order to prevent dielectric ceramic materials from being reduced, various techniques have been proposed (for example, see Japanese Unexamined Patent Application Publication Nos. 8-8137, 2001-97772, 2001-97773, 5-217793, 5-217794, 4-25005, and 11-278930). According to the reduction-preventing techniques of a dielectric ceramic material as mentioned above, manufacturing of a multilayer ceramic capacitor using nickel or the like as an interior electrode material can be performed.
In recent years, techniques for forming electronic circuits having a higher density have significantly advanced. Accordingly, a multilayer ceramic capacitor used for the electronic circuits as described above has been increasingly required to be miniaturized and to have a larger capacity. In addition, a multilayer ceramic capacitor may be used in some cases to isolate or buffer an electric source of a microprocessor which is operated at a high speed, and in this case, since an active electron element generates a large amount of heat while being operated at a high speed, a multilayer ceramic capacitor used around a microprocessor is required to have superior reliability in a high-temperature atmosphere.
Accordingly, even when the thickness of a dielectric ceramic layer forming a multilayer ceramic capacitor can be decreased, it has been desired that a dielectric ceramic material be realized which has a low dielectric loss, superior electrical insulating properties, and high reliability.
Although the dielectric ceramic materials disclosed in Japanese Unexamined Patent Application Publication Nos. 8-8137, 2001-97772, and 2001-97773 have a high relative dielectric constant, crystal grains in the ceramic are grown larger, and when the thickness of a dielectric ceramic layer is decreased, for example, to 3 μm or less, the number of crystal grains present in one dielectric ceramic layer is decreased, and as a result, a problem of degradation in reliability occurs.
Since the dielectric ceramic materials disclosed in Japanese Unexamined Patent Application Publication Nos. 5-217793, 5-217794, and 4-25005 use Ba—Si—Li or Ba—Si—B as a sintering auxiliary agent, problems may arise in that the properties of the dielectric ceramic material largely varies depending on firing conditions and in that the reliability in a high-temperature and high-humidity atmosphere is degraded.
According to the dielectric ceramic material disclosed in Japanese Unexamined Patent Application Publication No. 11-278930, a rare earth element which is added thereto is allowed to be primarily present in crystal grain boundaries so that the reliability by a high-temperature loading test is improved and, in addition, so that a higher relative dielectric constant is obtained. However, according to this dielectric ceramic material disclosed in Japanese Unexamined Patent Application Publication No. 11-278930, as are the materials disclosed in Japanese Unexamined Patent Application Publication Nos. 8-8137, 2001-97772, and 2001-97773, since crystal grains in the ceramic grow large, when the thickness of the dielectric ceramic layer is decreased, for example, to 3 μm or less, the number of crystal grains present in one dielectric ceramic layer is decreased, and as a result, a problem of degradation in reliability occurs.
Hence, an object of the present invention is to provide a dielectric ceramic capable of satisfying the desires described above while the above-described problems are dissolved and to provide a manufacturing method thereof.
Another object of the present invention is to provide a multilayer ceramic capacitor formed using the above-described dielectric ceramic.